The bonding of minerals by phosphoric acid or by phosphates is a well known process. For example, alumina or magnesium oxide can be bonded with mixtures of phosphates and phosphoric acid as reported by Salazar (U.S. Pat. No. 4,440,865). Other minerals such as vermiculite (Ekedahl et al, U.S. Pat. No. 2,919,202), clays (West et al, U.S. Pat. No. 2,660,536), aluminum silicates (Bartha et al, U.S. Pat. No. 4,303,449) and more generally refractory metal oxides (Herbst et al, U.S. Pat. No. 3,316,110) are reported as being bonded into useful products by the action of either phosphoric acid, phosphates of different types or mixtures of phosphates and sulfuric acid. Acid aluminum phosphate, designated also as monoaluminum orthophosphate (Al(H.sub.2 PO.sub.4).sub.3) is reported as binder for basic oxides of refractory nature: Adrian et al, Ger. Offen. 2,622,798; Stephen et al, U.S. Pat. No. 4,631,224; Noda et al, U.S. Pat. No. 3,958,582.
In examining the numerous substrates reported as being bonded with phosphates of different types, it was noted that micas such as muscovite or phlogopite were absent from the list of potential materials for the formation of phosphate bonded composites. This situation was unexpected since micas are known for their good thermal and electrical applicances as both refractory and electrical insulators or as "windows" in furnaces and, in the form of powder, as refractories in metallurgical operations. Attempts to bind mica with phosphoric acid as done with magnesium silicates have failed because such a material, particularly muscovite, is inert towards phosphoric acid as can be seen from Danas, Manual of Mineralogy, 17 ed. p. 468. Accordingly, because of the advantageous properties of mica, it would be desirable if a process could be developed to bind finely ground mica such as muscovite and phlogopite with an appropriate binding agent.